1. Field of the Invention
The present invention is directed to zeolite crystals, and preferably crystals of large pore zeolites, such as zeolite L, which have been washed to a pH within the range of about 9.4-10.0, to processes for accomplishing the desired zeolite washing, to catalytic reforming catalysts based on washed zeolite crystals, and catalytic reforming processes which utilize catalysts based on washed zeolite. Catalytic reforming catalysts made from zeolite washed into the desired 9.4 to 10.0 pH range exhibit activity, selectivity and activity maintenance which are significantly higher than catalysts using zeolites not washed.
2. Discussion of Background and Material Information
Catalytic reforming is a major petroleum refining process used to raise the octane rating of naphthas (C6 to C11 hydrocarbons) for gasoline blending. Catalytic reforming is also a principle source of aromatic chemicals (benzene, toluene, and xylenes) via conversion of paraffins and naphthenes to aromatics. The principle chemical reactions which occur during catalytic reforming include dehydrogenation of cyclohexanes to aromatics, dehydrocyclization of paraffins to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, isomerization of normal paraffins to branched paraffins, dealkylation of alkylbenzenes and hydrocracking of paraffins to light hydrocarbons, i.e., methane, ethane, propane, and butane. The latter reaction is undesirable and should be minimized since it produces light hydrocarbons not suitable for gasoline blending which have less value than gasoline fractions.
Reforming is carried out at temperatures of 800.degree. F. to 1000.degree. F., pressures of 50 to 300 psi, weight hourly space velocities of 0.5 to 3.0 and in the presence of hydrogen at hydrogen to hydrocarbon molar ratios of 1 to 10.
Reforming catalysts currently widely used in commercial reformers are platinum on an alumina substrate, and platinum plus a second promoting metal such as rhenium or iridium on alumina. These catalysts are bifunctional, i.e., the dehydrogenation reactions required in the reforming process are accomplished on the catalystic metal in the catalysts and the isomerization and cyclization reactions also required in reforming are accomplished on strong acid sites on the alumina catalyst support. Undesirable hydrocracking reactions which break C6+ paraffins down to lower molecular weight hydrocarbons and reduce selectivity to aromatics occur on the strong acid catalytic sites.
Alumina based reforming catalysts demonstrate reasonably high selectivities for converting C8+ paraffins and naphthenes to aromatics but are less satisfactory for aromatizing C.sub.6 to C.sub.8 paraffins; they hydrocrack more of the lower paraffins to low value fuel gas than they convert to aromatics.
New reforming catalysts are being developed which are significantly more active and selective for aromatizing C.sub.6 to C.sub.8 paraffins than alumina based catalysts. These new catalysts are zeolite based rather than alumina based. Zeolite based reforming catalysts are facile for aromatizing lower paraffins because they are monofunctional, i.e., they accomplish the isomerization reactions with great facility on the same catalystic metal active sites as the dehydrogenation and cyclization reactions. They do not require not contain strong acid sites which promote hydrogenolysis cracking reactions to accomplish isomerization. Moreover, certain zeolites have micropore dimension and configurations which sterically promote the desirable isomerization and dehydrocyclization reactions for C.sub.6 to C.sub.8 paraffins and repress undesirable hydrogenolysis cracking reactions. Accordingly, C.sub.6 to C.sub.8 paraffin selectivity to aromatics is high for these sterically favored zeolite catalysts. Zeolite which perform best as reforming catalysts substrates fall into the so-called "large pore" category which have pore diameters of 6 angstrom units or higher. The large pore zeolite L is a particularly good reforming catalysts substrate.
U.S. Pat. No. 4,448,891, COHEN, is directed to an improved reforming catalyst employing a zeolite L support provided by soaking the zeolite L in an alkali solution having a pH of at least 11 for a time and temperature effective to increase the period of time over which the catalytic activity of the catalyst is maintained, wherein the procedure involves washing the alkali soaked zeolite with water followed by repeated soakings in the zeolite solution for additional 18-hour periods with washing repeatedly thereafter until the pH of the zeolite water wash was at or below 10.5, followed by drying at 100.degree. C.
U.S. Pat. No. 4,544,539, and 4,593,133, WORTEL, are directed to zeolite related to zeolite L having certain characteristics wherein the process for preparation subsequent to separating the zeolite by centrifuging, involves washing four times with cold water prior to drying at 150.degree. C. In one procedure, disclosed in WORTEL '539, zeolite crystals were washed 5-6 times with cold water and the washings were decanted by centrifuging prior to drying in air for 16 hours at 150.degree. C.
U.S. Pat. No. 3,216,789, BRECK, relates to a process for producing synthetic zeolite which involves washing zeolite crystals, after the reactant mother liquor is filtered off, preferably with diluted water, until the effluent wash water, in equilibrium with the product, has a pH of between 9 and 12. This patent also discloses that as the zeolite crystals are washed, the exchangeable cation of the zeolite may be partially removed and is believed to be replaced by hydrogen cations. If the washing is discontinued when the pH of the effluent wash water is between about 10 and 11, the (K.sub.2 O+Na.sub.2))/Al.sub.2 O.sub.3 molar ratio of the crystalline product is disclosed as being approximately 1.0 but that excessive washing will result in a somewhat lower value for this ratio, while insufficient washing will leave a slight excess of exchangeable cations associated with the product.